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Audio performance advances in portables

-December 12, 2011

One of the consequences of the ongoing convergence of entertainment and computing products is increasing consumer expectations of audio and video performance on devices like smartphones and tablets.  To a large extent video quality is keeping up; despite their small screen sizes, these devices offer impressive video and graphics capabilities.  Their audio performance, especially that from the devices’ built-in transducers, is another story—a result of the significant physical size limitations imposed by the devices’ small and thin form factors.

This is being addressed on several fronts, including the devices’ built-in hardware and software/processing capabilities, as well as media quality itself—whether it’s streamed from the Internet or a local server, or from files stored on the device.  Companies like Dolby and Fraunhofer IIS, for example, offer codec solutions designed to improve sound from online streamed audio services while still minimizing bandwidth requirements.

With ever-increasing storage space availability, the size—and therefore quality—of on-board stored media, like music files, is now less of an issue than ever.  Some audiophiles are fond of lamenting on the quality of popular lossy compressed file formats like MP3, which are inherently technically inferior to CD-quality (or higher) formats.  This ignores the fact that encoding/decoding techniques have improved significantly since the first MP3 recordings appeared over 10 years ago. In fact codecs—such as the open-source LAME encoder—are now so good that well-encoded 192-kbps MP3 files have been shown to be virtually indistinguishable from CD quality in blind listening tests.     

And increasing storage capabilities and Internet bandwidth are making it easier than ever to create, store and distribute high-quality audio content.   Those audiophiles not satisfied with Amazon and iTunes' 256-kbps compressed file downloads can always purchase CDs, rip them using FLAC (Free Lossless Audio Codec), and stream CD-quality audio to their device using apps like iPeng (for iOS) or SqueezePlayer (for Android).

However, even with the highest-quality audio source material, there's only so much one can expect from the tiny speakers and microphones built into today's portable electronics.  But they too—along with the associated amplification and conversion electronics—are constantly being improved.  One recent example is STMicroelectronics' MP34DT01 (Download datasheetMP34DT01 datasheet) MEMS audio sensor omnidirectional stereo digital microphone that allows designers to place the microphone membrane closer to the acoustic port hole on the top of the package to increase performance without any penalty in size.

Fairchild Semiconductor recently announced a mobile audio initiative designed to address the challenge of improving audio performance in mobile devices, and followed up with the introduction of the FAB1200 (Download datasheetFAB1200 datasheet) Class-G headphone amplifier and FAB2200 (Download datasheetFAB2200 datasheet) audio subsystem. Other devices too, such as Wolfson Microelectronics' WM8918 ultra-low-power audio DAC and Maxim Integrated Products' MAX98089 (Download datasheetMAX98089 datasheet) stereo audio codec are designed to improve audio performance in portables.

And there's even a flat-panel speaker accessory cover for tablets using flat speaker solutions from HiWave Technologies. The SoundSleeve incorporates a pair of 2W audio exciters mounted on a lightweight honeycomb panel and is designed to be hidden behind the lid of a tablet computer sleeve.

Ultimately however, the most promise may lie with using software-based techniques to correct and enhance the sound quality in portables. These would include employing basic techniques such as equalization (EQ) and automatic gain control (AGC), as well as dynamic range compression (DRC) and even sophisticated psychacoustics and custom audio algorithms.

Wolfson Microelectronics' WM0010 programmable standalone audio DSP, for example, offers "off-the-shelf" algorithms in addition to allowing a manufacturer to integrate their own solutions. This applies as well to the company's WM5100 (Download datasheetWM5100 datasheet) audio SoC, which integrates three bespoke DSP cores, allowing manufacturers to use their own or licensed algorithms.

Texas Instrument
Enhancing the sound from small (and closely spaced) speakers on portable electronics is being addressed by devices like Texas Instrument's LM48901 configurable spatial processor, which uses loudspeaker array technology to produce an immersive end-user audio experience.

Finally, Texas Instruments' LM48901 (Download datasheetLM48901 datasheet) configurable spatial processor aims at addressing audio soundstage limitations in multispeaker portables, like tablets (see Figure). The device, along with its companion software, allows designers to easily design spatial audio enhancement for applications with up to 16 loudspeakers to create an immersive listener experience.

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